Polishing, as a form of lapping, generally involves an application with force of an abrasive substance against a surface of an object to be polished so as to obtain a relatively smooth finish. Contrasted with grinding which is the other form of lapping and which is primarily concerned with abrading away a substantial portion of a surface and prominent irregularities therein, polishing is concerned with removal of minute or even microscopic irregularities.
Polishing and grinding are both typically conducted utilizing lapping compositions comprised of relatively small inorganic grit particles that serve as an abrasive agent and a carrier medium for same in which the particles are contained. The size of the grit particles used basically determines the smoothness of the finish obtained with 10 microns being the usual division size between grinding and polishing operations. Precision polishing operations for such materials as optical components, plastics, metals, gemstones, semiconductor components, and the like typically involve the use of even finer abrasive grit particles and present particular problems previously unsolved.
Typically, previous lapping compositions for use in connection with precision polishing have involved suspensions of very fine abrasive grit particles (less than about 5 microns in size). However, it has been extremely difficult to maintain the abrasive particles in homogeneous suspension in the carrier medium, particularly considering the relatively small amount of abrasive particles present compared to the large amount of medium in such compositions.
This problem has been particularly noted with using water as a medium; which, while being a common medium due to its ready availability and cheapness, has produced a suspension in which the abrasive particles mixed therein rapidly settle out of suspension and thereafter cake at the bottom of the composition container. Also, water based suspensions have exhibited a relatively fast loss of moisture during use necessitating frequent additions of further fluid during polishing operations to maintain the desired properties of the polishing composition on the polishing lap. Oil has sometimes been used as a polishing suspension carrier medium but has not been particularly desirable due to attendant removal problems from the lap and the work piece and the tendency of oil to flash during polishing operations, as well as nonpermanency of suspension.
Recently, fine polishing compositions have been introduced which utilize di-hydroxy glycols (such as ethylene glycol) as the primary component or base of the carrier medium. In connection with the glycol base, which is a glycol/water mixture with there being more glycol than water by overall weight-percent, a small amount of carboxy polymethylene polymer of high molecular weight has been introduced into the carrier to act as a composition thickener and better keep the abrasive particles in suspension. It has been necessary in making such compositions to add a neutralizing agent to the acidic polymer in order to achieve proper thickening of the carrier.
Such glycol based compositions admittedly have been superior to the preceding water based compositions in terms of permanency of suspension and resistance to drying out but have nonetheless not fully met the preexisting deficiencies and have indeed exhibited further undesired properties. For example, glycols generally are toxic substances so that human contact in polishing operations utilizing glycol based polishing compositions have had to be carefully carried out and monitored in a controlled environment lest repeated contact lead to undesirable results. The toxic nature of glycols further has made the ultimate disposal of waste portions of glycol based compositions a complicated and necessarily expensive process.
Furthermore, although less prone to rapidly dry out than the water based compositions, the glycol based polishing compositions still dry out on the lap during normal polishing operations. This presents a problem particularly when extended precision polishing operations are involved, and undesired additions of liquid are required to complete the polishing operations. Also, glycol based polishing compositions have exhibited a tendency to react with and stain and disfigure certain polished surfaces when not flushed and washed off immediately. This property of partial activity appears to explain why glycol based suspensions have been often unusually difficult to remove from polished metal surfaces.
In addition to the above deficiencies of existing polishing compositions noted relating to the liquid suspension media, further short-comings have been recognized in connection with the polishing abrasive used. For example, it has been common to use an aluminum oxide or alumina powder abrasive of nominal 3 micron size or less in typical fine polishing operations. However, it has been observed that agglomerates are contained in such aluminas that substantially increase the effective particle size of the abrasive to much larger than desired. The result is that use of such an agglomerated alumina may yield a substantially scratched surface after polishing that then has had to be repolished using a still finer abrasive in order to obtain a relatively scratch-free surface.